Figure 1.

Reproductive modes of triploid vertebrates. Shown are the parental, gametic and offspring genomes (rows) under different reproductive modes (columns). A, B: genomes of different parental species. Bold coloured symbols indicate clonally transmitted copies, while thin black symbols with superscripts indicate different (recombined) copies. True parthenogenesis: clonal (males absent), restricted to reptiles [7,8]; Sperm-dependent parthenogenesis (i.e. gynogenesis): clonal, embryogenesis requires trigger from allospecific sperm that is not incorporated (rare ‘paternal leakage’ might incorporate subgenomic amounts of paternal DNA), occurs in teleost fishes and urodelan amphibians [9]; Kleptogenesis: females acquire full or partial genomes from their mates by a not fully understood mechanism, allowing them to purge genomes from deleterious alleles (here BB); described from urodelan amphibians [10]; Unnamed form of hybridogenesis: clonal diploid eggs are fertilized by sperm from a recombining sexual species that can be diploid or triploid (as in meiotic hybridogenesis); occurs in anuran amphibians and teleost fishes [11–13]; Meiotic hybridogenesis: may occur in triploid males and/or females; found in teleost fishes and anuran amphibians [14,15]; ploidy elevation of the diploid offspring, which might produce diploid hybrid gametes, can occur in the next generation (becoming then e.g. ABB′) to restore triploidy (similar to preceding form of hybridogenesis); Pre-equalizing hybrid meiosis: occurring in Batura toads: Both sexes are triploid and exhibit Mendelian segregation and recombination in the B genomes (equivalent to NOR+; this paper), while the A genome (i.e. NOR−) is clonally transmitted by the mother.